David R. Stapells

Intracerebral Sources of Human Auditory Steady-State Responses

The objective of this study was to localize the intracerebral generators for auditory steady-state responses. The stimulus was a continuous 1000-Hz tone presented to the right or left ear at 70 dB SPL. The tone was sinusoidally amplitude-modulated to a depth of 100% at 12, 39, or 88 Hz. Responses recorded from 47 electrodes on the head were transformed into the frequency domain. Brain electrical source analysis treated the real and imaginary components of the response in the frequency domain as independent samples. The latency of the source activity was estimated from the phase of the source waveform. The main source model contained a midline brainstem generator with two components (one vertical and lateral) and cortical sources in the left and right supratemporal plane, each containing tangential and radial components. At 88 Hz, the largest activity occurred in the brainstem and subsequent cortical activity was minor. At 39 Hz, the initial brainstem component remained and significant activity also occurred in the cortical sources, with the tangential activity being larger than the radial. The 12-Hz responses were small, but suggested combined activation of both brainstem and cortical sources. Estimated latencies decreased for all source waveforms as modulation frequency increased and were shorter for the brainstem compared to cortical sources. These results suggest that the whole auditory nervous system is activated by modulated tones, with the cortex being more sensitive to slower modulation frequencies.

Thresholds for auditory brain stem responses to tones in notched noise from infants and young children with normal hearing or sensorineural hearing loss.

Objective To assess the accuracy of threshold estimates determined using the auditory brain stem responses (ABRs) to brief tones presented in notched noise in a group of infants and young children with normal hearing or sensorineural hearing loss (SNHL). Design The thresholds for ABRs to brief duration 500, 2000, and 4000 Hz tones presented in notched-noise masking were evaluated in infants and young children with normal hearing (N = 34) or SNHL (N = 54). Tone-evoked ABR thresholds were compared with behavioral thresholds obtained at follow-up audiologic assessments, for a total of 220 comparisons. Results ABR thresholds for the infants with bilateral normal hearing were 23.6,12.9, and 12.6 dB nHL for 500, 2000 and 4000 Hz, respectively. Most (92 to 100%) infants with normal hearing showed ABRs to 30 dB nHL tones. Across all subjects (i.e., those with normal hearing and those with impaired hearing), high (20.94) correlations were found between the ABR and behavioral thresholds. The mean differences between ABR (dB nHL) and behavioral (dB HL) thresholds across all subjects were 8.6, -0.4, and -4.3 dB for 500, 2000, and 4000 Hz, respectively. Overall, 98% of the ABR thresholds were within 30 dB of the behavioral thresholds, 93% were within 20 dB, and 80% were within 15 dB. Conclusions These threshold results for the ABR to brief tones in notched noise obtained for infants and young children are similar to those obtained in similar studies of adults. The technique may be used clinically with reasonable accuracy to estimate pure-tone behavioral thresholds in infants and young children who are referred for diagnostic threshold ABR testing.

Thresholds determined using the monotic and dichotic multiple auditory steady-state response technique in normal- hearing subjects

Auditory steady-state responses (ASSRs) were elicited by presenting single or multiple, 77-105 Hz amplitude-modulated 0.5, 1, 2, and 4 kHz tones to one or both ears. Objectives of this study were to (i) replicate and extend previous multiple ASSR studies in a quiet double-walled sound booth, and (ii) discover differences (if any) between thresholds assessed in monotic and dichotic conditions, which ranged between 15 and 22 dB SPL. The present studys behavioural and ASSR thresholds are 0-10 dB lower (better) than results of previous monotic studies. Further, there are no significant differences in ASSR thresholds between dichotic and monotic stimulus conditions. Therefore, dichotic multiple AM tone stimulation does not produce a change in the ASSR that affects threshold estimation in a clinically significant manner. Thus, at least for detecting normal hearing, the dichotic multiple ASSR technique is a feasible method for estimating hearing thresholds that would substantially reduce recording time compared to conventional single-stimulus techniques.

Children's Event-Related Potentials of Auditory Selective Attention Vary With Their Socioeconomic Status

Past research suggests a link between socioeconomic status (SES) and brain processes in children, but direct evidence from neuroimaging is scarce. The authors investigated the relationships among SES, performance, and the neural correlates of auditory selective attention, by comparing event-related potentials (ERPs) in lower- and higher-SES preadolescent children during a task in which they attended to two types of pure tones but ignored two other types. Our hypothesis was that, at comparable performance levels, higher-SES children ignore distracters (the unattended, irrelevant tones) while lower-SES children attend equally to distracters and to targets (the attended, relevant tones). The authors found that ERP waveform differences between attended and unattended tones (Nd, difference negativity) were significant in the higher-SES but not in the lower-SES group. However, the groups did not differ in reaction times or accuracy. Electroencephalographic power analysis revealed a differential pattern of theta activity concomitant with irrelevant tones for the two groups, indicating that although they performed similarly the children from these groups recruited different neural processes. Lower-SES children, the authors suggest, deployed supplementary resources to also attend to irrelevant information.

Normal hearing thresholds for clicks

This paper evaluates the normal hearing thresholds for clicks and assesses the effects on these thresholds of varying the duration of the listening period and the presentation rate, polarity, and symmetry of the clicks. There were no significant changes in thresholds as the listening period decreased from 2s to 300 mg. There was, however, a 2.5 dB increase in threshold at the listening period decreased from 300 to 100 ms. Increase stimulus presentation rate from 5 to 80/s decreased threshold 4.5 dB per tenfold change in rate. There were no significant differences in threshold between rarefaction and condensation clicks. The average threshold obtained from 40 normal young adults using 100-microseconds square-wave clicks presented through a TDH-49 earphone at 10/s was 36.4-dB peak SPL or 29.9 peak equivalent SPL. Neither peak SPL nor peak equivalent SPL measurements gave consistent thresholds for clicks with different degrees of symmetry. A root-mean-square measure of the pressure over the initial millisecond-SPL(1 ms)-gave a threshold of 25.6 dB. This SPL(1ms) measure of threshold proved to be far more consistent for clicks with different degrees of symmetry than either the peak SPL or the peak equivalent SPL measures.

Estimation of the Pure-Tone Audiogram by the Auditory Brainstem Response: A Review

This review paper briefly considers how stimulus, noise masking and recording parameters affect the frequency and place specificity of auditory brainstem responses (ABRs) to air- and bone-conducted stimuli. Issues concerning the use of clicks for ABR threshold estimates will first be presented, followed by results for tone-evoked ABR thresholds and how well they predict the pure-tone behavioral audiogram. Noise-masking options (e.g. high-pass noise, notched noise and white noise) to improve the frequency specificity of tone-evoked ABRs, which are now available on clinical ABR units, will also be discussed. The goal of this article is to demonstrate that ABRs to tonal stimuli can be successfully recorded in most clinical environments and can provide reasonably accurate estimates of 500- to 4000-Hz pure-tone behavioral thresholds in infants, children and adults. Specific parameters and protocols for obtaining frequency-specific ABR threshold responses are provided.

Auditory steady-state response thresholds of adults with sensorineural hearing impairments.

This study evaluated the use of multiple auditory steady-state responses (ASSRs) to estimate the degree and configuration of behavioral audiograms of subjects with sensorineural hearing impairments. Place specificity of the multiple-ASSR method was also assessed. Multiple amplitudemodulated (77–105 Hz) tones (500, 1000, 2000 and 4000 Hz) were simultaneously presented to one ear. The results showed that, on average, multiple-ASSR thresholds were 14±13, 8±9, 10±10 and 3±10 dB above behavioral thresholds for 500, 1000, 2000 and 4000 Hz, respectively. Behavioral and multiple-ASSR thresholds were significantly correlated (r =0.75–0.89). There were no significant differences between behavioral and multiple-ASSR measures of the audiogram configuration. In subjects with steep-sloping ≥30 dB/octave) hearing losses, multiple-ASSR thresholds did not underestimate behavioral thresholds, revealing good place specificity. These results indicate that the multiple-ASSR method provides good estimates of the degree and configuration of hearing in individuals with sensorineural hearing impairments. Este estudio evaluó el uso de las repuestas auditivas de estado estable (ASSR) para estimar el grado y la configuración de la audiometria conductual en sujetos con hipoacusia sensorineural. También se evaluó la especificidad tonal del método de ASSR múltiple. Se presentaron tonos múltiples de amplitud modulada (77–105 Hz) en 500, 1000, 2000 y 4000 Hz en forma simultánea para cada oído. Los resultados muestran que en promedio, los umbrales de ASSR-múltiple son 14±13, 8±9, 10±10 y 3±10 dB por encima del umbral conductual para las frecuencias 500, 1000, 2000 y 4000Hz respectivamente. No hubo una diferencia significativa (r = 0.75–0.89) entre los umbrales conductuales y los umbrales por ASSR-múltiple en la configuración del audiograma. En sujetos con curvas de perfil descendente abrupto (≥30 dB/octave), los umbrales obtenidos por ASSR-múltiple no subestimaron los umbrales conductuales y demostraron buena relación de especificidad frecuencial. Estos resultados indican que el método ASSR-múltiple proporciona un buen estimado del grado y la configuración de la audición de los individuos con hipoacusia sensorineural.

The effects of decreased audibility produced by high-pass noise masking on cortical event-related potentials to speech sounds /ba/ and /da/

This study investigated the effects of decreased audibility produced by high-pass noise masking on cortical event-related potentials (ERPs) N1, N2, and P3 to the speech sounds /ba/and/da/presented at 65 and 80 dB SPL. Normal-hearing subjects pressed a button in response to the deviant sound in an oddball paradigm. Broadband masking noise was presented at an intensity sufficient to completely mask the response to the 65-dB SPL speech sounds, and subsequently high-pass filtered at 4000, 2000, 1000, 500, and 250 Hz. With high-pass masking noise, pure-tone behavioral thresholds increased by an average of 38 dB at the high-pass cutoff and by 50 dB one octave above the cutoff frequency. Results show that as the cutoff frequency of the high-pass masker was lowered, ERP latencies to speech sounds increased and amplitudes decreased. The cutoff frequency where these changes first occurred and the rate of the change differed for N1 compared to N2, P3, and the behavioral measures. N1 showed gradual changes as the masker cutoff frequency was lowered. N2, P3, and behavioral measures showed marked changes below a masker cutoff of 2000 Hz. These results indicate that the decreased audibility resulting from the noise masking affects the various ERP components in a differential manner. N1 is related to the presence of audible stimulus energy, being present whether audible stimuli are discriminable or not. In contrast, N2 and P3 were absent when the stimuli were audible but not discriminable (i.e., when the second formant transitions were masked), reflecting stimulus discrimination. These data have implications regarding the effects of decreased audibility on cortical processing of speech sounds and for the study of cortical ERPs in populations with hearing impairment.

The effects of broadband noise masking on cortical event-related potentials to speech sounds /ba/ and /da/

Objective: To systematically investigate in normal‐hearing listeners the effects of decreased audibility produced by broadband noise masking on the cortical event‐related potentials (ERPs) N1, N2, and P3 to the speech sounds /ba/ and/da/. Design: Ten normal‐hearing adult listeners actively (button‐press response) discriminated the speech sounds /ba/ and /da/ presented in quiet (no masking) or with broadband masking noise (BBN), using an ERP oddball paradigm. The BBN was presented at 50, 60, and 70 dB SPL when speech sounds were presented at 65 dB ppe SPL and at 60, 70 and, 80 dB SPL when speech sounds were presented at 80 dB ppe SPL. Results: On average, the 50, 60, 70, and 80 dB SPL BBN maskers produced behavioral threshold elevations of 18, 25, 35, and 48 dB (average for 250 to 4000 Hz), respectively. The BBN maskers produced significant decreases (relative to quiet condition) in ERP amplitudes and behavioral discriminability. These decreases did not occur, however, until the noise masker intensity (in dB SPL) was equal to or greater than the speech stimulus intensity (in dB ppe SPL), that is, until speech to noise ratios (SNRs) were ≤0 dB. N1 remained present even after N2, P3, and behavioral discriminability were absent. In contrast to amplitudes, ERP and behavioral latencies showed significant decreases at higher (better) SNRs. Significant latency increases occurred when the noise maskers were within 10 to 20 dB of the stimuli (i.e., SNR ≤ 20 dB). The effects of masking were greater for responses to /da/ compared with /ba/. Latency increases occurred with less masking for N1 than for P3 or behavioral reaction time, with N2 falling in between. Conclusions: These results indicate that decreased audibility as a result of masking affects the various ERP peaks in a differential manner and that latencies are more sensitive indicators of these masking effects than are amplitudes.

Technical aspects of brainstem evoked potential audiometry using tones.

The brainstem response to brief tones contains a large vertex-positive component. If high-pass filter settings above 20 Hz are used, particularly with high rolloff slopes, the morphology of the response changes so that a vertex-negative wave becomes the most prominent component of the response. The amplitude of the response is unaffected by stimulus presentation rates of up to 35/sec. Tones with longer rise times have greater frequency specificity, but at rise times of greater than 5 msec, the brainstem response becomes very small. At high intensities, regardless of the rise time, the response to tones is not completely frequency specific, and notched noise masking should be used to obtain frequency-specific responses.